1. Field of the Invention
The present invention relates to a method for fabricating a sintered duplex nuclear fuel pellet, and more particularly to a method for fabricating a sintered duplex nuclear fuel pellet consisting of a cylindrical inner portion composed of UO2—Gd2O3 and an annular outer portion composed of UO2—Er2O3 or UO2, which is capable of preventing the occurrence of cracks due to the difference in the densification rate between the two portions upon sintering.
2. Description of the Related Art
Generally, the most widely used sintered pellet as a nuclear fuel is a sintered uranium dioxide (UO2) pellet. The sintered uranium dioxide pellet mainly has a height of about 10 mm and a diameter of about 8 mm in a cylindrical shape for practical application. The sintered UO2 pellet contains 1-5% by weight of U235 and produces nuclear fission energy while U235 decays by the action of neutrons during use in a nuclear reactor. In addition to the sintered UO2 pellet, a sintered burnable absorber pellet containing neutron absorbing materials, such as gadolinium (Gd) or erbium (Er), is used in a nuclear reactor to control the numbers of neutrons.
The sintered burnable absorber pellet has generally the same size as the sintered UO2 pellet. Gd or Er is uniformly distributed throughout the whole sintered UO2 pellet. The sintered burnable absorber pellet is commonly represented by (U,Gd)O2 or (U,Er)O2, and is fabricated in accordance with the following procedure.
First, UO2 powder is mixed with Gd2O3 or Er2O3 powder, and pulverized to prepare a UO2—Gd2O3 powder or UO2—Er2O3 powder. At this time, the Gd2O3 content and the Er2O3 content are limited to about 15% or less and 4% by weight or less, respectively.
Thereafter, the pulverized powder is subjected to compression molding to produce a compact, which is sintered by heating under a reducing gas atmosphere at 1,600-1,800° C. for 2-4 hours. The density of the compact is about 50-70% of the theoretical value, and that of the sintered pellet is about 95% of the theoretical value. During the sintering, Gd or Er is dissolved into the lattice structure of UO2. The reducing gas may be hydrogen gas, or a mixed gas of hydrogen gas with at least one gas selected from water vapor, inert gases and carbon dioxide.
Korean Patent No. 0281169 (published on Nov. 15, 2000), issued to Korea Atomic Energy Research Institute, describes that a duplex structure consisting of a cylindrical inner portion and an annular outer portion composed of different materials is advantageous over a structure using only a sintered (U,Gd)O2 or (U,Er)O2 pellet as a sintered burnable absorber pellet, because the duplex structure exhibits improved nuclear performance in a nuclear reactor. FIG. 1 is a schematic view showing the structure of a conventional sintered duplex nuclear fuel pellet 10 consisting of a cylindrical inner portion 1 and an annular outer portion 2.
The sintered duplex burnable absorber nuclear fuel pellet is fabricated by charging different nuclear fuel powders into the inner and outer portions, molding the portions to produce a duplex compact, and sintering the duplex compact.
However, the sintering process may have a number of problems based on the fact that the inner portion and the outer portion are composed of different materials. That is, the annular outer portion is composed of a mixed powder UO2—Er2O3 and the cylindrical inner portion is composed of a mixed powder UO2—Gd2O3. During the sintering process (also, referred to “densification”) for increasing the density of the compact while contracting, a large difference in the densification rate of the cylindrical inner portion and the annular outer portion composed of different materials is caused, which generates an undesirable internal stress at the interface between both portions. Consequently, serious interstices or cracks occur at the interface of the final sintered duplex nuclear fuel pellet.
In an effort to overcome these problems, U.S. Pat. No. 4,678,629 suggests a method comprising sintering a cylindrical inner portion (UO2—Gd2O3) and an annular outer portion (UO2) separated from each other, and fitting the sintered cylindrical inner portion into the sintered annular outer portion. However, since this method requires precise fabrication and processing of the sintered portions, it is not an effective approach for practical application.
Alternatively, M. Fisher (J. Nucl. Mater., 138, 242-247 (1986)) reported a method for fabricating a sintered duplex pellet consisting of a ThO2 outer portion and a UO2 inner portion through two-step sintering. According to this method, the UO2 inner portion is first sintered, fitted into a ThO2 compact, and further sintered to fabricate a sintered pellet having no cracks. However, this method further involves fitting of the sintered UO2 inner portion into the ThO2 compact, which renders the overall processes more complicated. In addition, since this method does not suggest a basic solution to a large difference in the densification rate between the cylindrical inner portion and the annular outer portion, the occurrence of cracks still cannot be avoided, resulting in deterioration of the strength of the sintered pellet and damage during handling. In particular, the thermal conductivity is reduced, causing a reduction in the operational performance in a nuclear reactor.
Thus, there is a need in the art for a method for fabricating a sintered duplex burnable absorber nuclear fuel pellet free from cracks derived from the difference in the densification rate between a cylindrical inner portion and an annular outer portion composed of different materials upon sintering.